Copper bis(thiosemicarbazone) Complexes with Pendent Polyamines: Effects of Proton Relays and Charged Moieties on Electrocatalytic HER

Abstract

A series of new bis(thiosemicarbazonato) Cu(II) complexes with pendent polyamines, diacetyl-(N,-dimethylethylenediaminothiosemicarbazonato)-(N'-methyl-3-thio-semicarbazonato)butane-2,3-diimine)-copper(II) (Cu-1), diacetyl-bis (N-dimethylethylenediamino-3-thiosemicarbazonato)butane-2,3-diimine)-copper(II) (Cu-3), and their cationic derivatives Cu-2 and Cu-4, have been synthesized and fully characterized by spectroscopic, electrochemical, and X-ray diffraction methods. Complexes Cu-1-Cu-4 are analogues of Cu(ATSM), which contains a similar N 2 S 2 donor core with terminal non-coordinating amines. Substitution of the methyl group(s) of the terminal amines of H 2 ATSM with N,N-dimethylethylenediamine followed by alkylation generates a charged quaternary amine in the ligand framework. The charged site tunes the redox potentials of the complexes with minimal changes in their physical and electronic properties. The HER activity of all four copper complexes were evaluated in acetonitrile with glacial acetic acid. All of the complexes have lower HER overpotentials than Cu(ATSM), which is attributed to charge effects. The pendent amines of Cu-1 and Cu-3 have the lowest HER overpotential as the pendent tertiary amine also serves as a proton relay to enhance proton rearrangement under catalytic conditions. Complex Cu-3 showed the highest activity with a TOF of 12 × 10 3 s À 1 , an overpotential of 0.65 V, and faradaic efficiency of 100 %.

Ligand-Centered Hydrogen Evolution with Ni(II) and Pd(II)DMTH

“…The CVs of NiL 1 and PdL 1 both exhibit curve crossing when the concentration of HNEt 3 + exceeds 80 equiv. Crossing behavior occurs when the kinetics in the forward sweep are slower than in the reverse sweep and may be indicative of film formation. , Film formation has frequently been reported with M­(BTSC) HER catalysts but there are no prior reports of the curve crossing phenomenon. ,,,− Sequential CV scans for both NiL 1 and PdL 1 under saturating conditions of HNEt 3 + showed an anodic shift in the catalytic wave confirming film formation (Figures S21–S25). The CVs of NiL 1 and PdL 1 with acetic acid and HDBU + also showed an anodic shift in the catalytic wave consistent with film formation after multiple scans, although curve crossing was not always observed.…”

“…Redox-active ligands are of special interest in HER because the ligand can act as an electron reservoir and facilitate two-electron activity with earth-abundant, cost-efficient, first-row transition metals in molecular catalysts. − For several years, we and others have evaluated Zn, , Cu, − Ni, − Pd, and Co , complexes with redox-active bis-thiosemicarbazone (BTSC) ligands (Figure ) as electrocatalysts for the HER. The highly modular nature of BTSCs allows tuning of ligand electronics and basicity.…”

“…This cooperativity between the metal and noninnocent ligand can lead to different H 2 evolution pathways based on the specific metal–ligand combination. As shown for M­(BSTC)­s, pathways include: (1) metal-assisted ligand-centered, where the metal acts as an electron reservoir for bond making/breaking reactions at the ligand; − (2) ligand-assisted metal-centered, where the ligand is an electron reservoir supporting a metal hydride intermediate; , and (3) ligand-centered, where the ligand alone is responsible for the activity with the metal absent or in a structural role. ,− , For ZnL 1 , the reaction follows a ligand-centered pathway as the metal is redox inactive. Herein, we report the synthesis and characterization of nickel­(II) and palladium­(II) derivatives, NiL 1 and PdL 1 , and their respective HER activity in acetonitrile with three different acids.…”

Redox-Active Ligands in Electroassisted Catalytic H+ and CO2 Reductions: Benefits and Risks

“…Initially poorly basic positions then become proton-responsive and may therefore function as proton relays. Among the catalytic systems that typically promote ligand protonation upon reduction, a number of studies have been dedicated to metal dithiolene ,− and metal thiosemicarbazone ,− complexes. The former initiated early works on redox-active ligands, due to the inability of conventional oxidation-state descriptors to provide a satisfying depiction of their electronic structures …”

Nickel Complexes and Carbon Dots for Efficient Light‐Driven Hydrogen Production

“…Among these catalysts, we can distinguish Ni‐based complexes with thiosemicarbazone ligands [26–30] . These type of ligands have already been studied and proved to be redox‐active [31–39] . Furthermore, the presence of S‐ and N‐atoms in the ligand permits the protonation of the ligand and can serve as proton relays [40–42] .…”